PHONON ANOMALIES IN HIGH Tc SUPERCONDUCTORS RELATED TO CHARGE STRIPE ORDER

It is a wideheld belief that the holes doped into the CuO2 planes of the cuprate superconductors tend to segregate into stripes that separate antiphase antiferromagnetic domains. However, solid evidence for such stripe order was obtained so far only for the particular compound La1.48Nd0.4Sr0.12CuO4 (LNSCO) and the closely related one La1.875Ba0.125CuO4. In these two compounds, nuclear and magnetic superlattice peaks were observed indicative of static stripe order [1]. These compounds are special in that their doping level is about 1/8 and in that their low temperature structure provides a pinning potential for the charge stripes. We note that static stripe order seems to suppress superconductivity and therefore, stripe order is assumed to be dynamic in the superconducting cuprates. Inelastic neutron scattering may be used to look for signatures of dynamic stripe both in the charge and in the spin channel. Here, we focus on the charge channel. Although neutrons cannot observe directly an inhomogeneous charge distribution, they may do so indirectly by looking at the atomic displacements induced by the inhomogeneous charge. More precisely, they can probe atomic vibrations and those vibrations having a displacement pattern closely related to charge stripe order should manifest themselves by an anomalous behaviour. Since an inhomogeneous charge distribution will modulate the Cu-O bond length, stripe-related phonon anomalies are expected to be strongest in Cu-O bond-stretching vibrations. Several neutron scattering investigations carried out at LLB on a number of doped and undoped high Tc compounds have indeed shown that doping induces pronounced anomalies in just those phonon branches associated with plane-polarized Cu-O bond-stretching vibrations whereas nearly all other modes show little change upon doping [2]. More precisely, doping induces a pronounced frequency renormalization of zone boundary modes in the (100) direction (Fig. 1) and, to a smaller extent, also in the (110) direction. Figure 1. Dispersion of longitudinal Cu-O bondstretching vibrations along the (100)-direction determined by inelastic neutron scattering at T = 10 K on La2-xSrxCuO4 for insulating (x = 0), underdoped (x = 0.1), optimally doped (x = 0.15) and overdoped (x = 0.3) samples.